1. Field of the Invention
The present invention relates to axial and centrifugal pumps and more particularly to such pumps which utilize shrouded inducers. The present invention provides a means for avoiding cavitation damage resulting from the recirculation of a fluid flow about an outer periphery of the shroud on such a pump.
2. Description of the Prior Art
It has been found that the addition of a shroud to an inducer eliminates the formation of vortices at or about the tips of the inducer blades and the corresponding cavitation damage to the inducer blade which would result from such vortices. The use of a shroud however, introduces new problems, namely, a portion of the fluid downstream of the inducer (high pressure discharge end) tends to recirculate about an outer periphery of the shroud and re-enter the main flow of fluid just upstream (low pressure, inlet end) of the inducer shroud and blades. As the recirculating, high-pressure fluid emerges from behind the shroud it creates vortices which impinge upon the more radially outward, inlet edges of the inducer blades. These vortices create an erosive action upon the afflicted portion of the blades and shroud which will result in a loss in efficiency and structural integrity as previously described with respect to an unshrouded blade tip. Thus, the provision of a shroud to avoid the problems associated with tip vortices is compromised by the problems associated with vortices shed at the forward or inlet edge of the shroud.
In attempts to overcome the problems associated with the shrouded inducer, it has been customary to provide labyrinth seals about the outer periphery of the inducer shroud to minimize the amount of fluid recirculated over the outer surface of the shroud. A labyrinth seal, however, only reduces the amount of flow it does not eliminate it. Further, labyrinth seals tend to lose their effectiveness with time due to wear; particularly in pumps where vibration and thermal transients subject the seal to any degree of rubbing. It has been proposed to make an extensive use of labyrinth seals for example, substantially throughout the length of the shroud (see U.S. Pat. No. 2,984,189). Obviously, however, such extensive use is impractical and costly.
U.S. Pat. No. 3,221,661 describes the centrifugal pump including a housing having an inlet section and a peripheral outlet section. An impeller is rotatably mounted in the housing and is provided with a shroud section. The shroud section is cylindrical adjacent to the inlet section and divergent adjacent to the peripheral outlet section. A terminal involute section is attached the interior of the shroud section and is located in the divergent portion of the shroud section. The patent further describes a means for introducing any fluid bypassing the shroud section into the pump housing adjacent the inlet section in a direction toward the impeller blades. A disadvantage of this device is that any fluid bypassing or recirculating about the outer periphery of the shroud is at an elevated pressure and is reintroduced into the pump in a region of low pressure, thus creating the potential problem of cavitation.
U.S. Pat. No. 4,449,888 describes a free-spool inducer pump. The pump comprises a housing, an impeller rotatable in the housing to discharge fluids centrifugally outwardly, a rotary spool having an inducer section through which intake fluid passes to the impeller and a rotary diffuser associated with the rotary spool. The diffuser has driving vanes located at the inlet of the diffuser and outwardly of the impeller periphery for rotatively driving the spool in response to outward flow of discharged fluid against and between the vanes. A disadvantage of this apparatus is its complexity and the associated cost of its manufacture.
Obviously, there still remains the need for a means of constructing a shrouded inducer which will substantially reduce, if not eliminate, the problems associated with vortices emanating from the shroud and which does not require costly and complex configurations.
3. Objects of the Invention
It is an object of the present invention to provide an improved shrouded inducer which will substantially minimize or eliminate any cavitation damage from fluid recirculating about an outer surface of the shroud.
It is another object of the present invention to provide a shrouded inducer which does not require an extensive number of labyrinth seals.
Yet another object of the present invention is to provide a shrouded inducer which suffers no cognizable degree of cavitation damage either from tip vortices or from vortices shed by fluid being recirculated about an outer periphery of the inducer shroud.
Still another object of the present invention is to provide a shrouded inducer which is readily fabricable without undue cost.